1. Field of the Invention
The present disclosure relates to a light beam emission apparatus configured to detect a light quantity of a light beam and to control the light quantity of the light beam based on the detected result, and an image forming apparatus including the light beam emission apparatus.
2. Description of the Related Art
An image forming apparatus, such as a copying machine and a printer, using an electrophotographic method forms an electrostatic latent image on a photosensitive member by scanning the photosensitive member with a light beam, such as a laser beam, and develops the latent image with tonner. As for an apparatus that emits a laser beam for exposing the photosensitive member, a light beam emission apparatus, such as an optical scanning apparatus, has been used.
The above optical scanning apparatus converts alight flux from a semiconductor laser (light source) into a substantially parallel light flux, and then deflects the light flux by a rotating polygonal mirror (hereinafter referred to as a polygon mirror). The laser beam deflected by the rotating polygon mirror substantially linearly scans the photosensitive member.
The optical scanning apparatus in recent years exposes a photosensitive member with a plurality of laser beams emitted from a plurality of light emitting points, to deal with a high image forming rate and high image resolution. Particularly, in a vertical cavity surface emitting laser (VCSEL), a plurality of light emitting points can be easily arrayed and thus, it has been proposed to use the VCSEL as a light source of the optical scanning apparatus.
The optical scanning apparatus detects a light quantity from a light source by an optical sensor and controls the quantity of a laser beam emitted from the light source based on the detected light quantity. The VCSEL is different from an edge-emitting semiconductor laser and does not have a back surface laser beam corresponding to a laser beam guided to a photosensitive member. Thus, to control the light quantity of a laser beam emitted from the VCSEL, Japanese Patent Application Laid-Open No. 2006-91157 and Japanese Patent Application Laid-Open No. 2006-259098 each discuss an optical scanning apparatus that separates a part of the laser beam emitted to the photosensitive member by a half mirror and detects the light quantity by guiding the separated laser beam to an optical sensor. A light quantity ratio between the light quantity of a laser beam passing through a half mirror and reaching a photosensitive member and the light quantity of a laser beam guided to an optical sensor is uniquely determined by the reflectance of the half-mirror. Therefore, by controlling the light quantity of the laser beam emitted from the VCSEL according to the received light quantity of the optical sensor, the optical scanning apparatus can control the light quantity of the laser beam reaching the photosensitive member to become a target light quantity.
However, the optical scanning apparatus discussed in Japanese Patent Application Laid-Open No. 2006-91157 has the following issue. As illustrated in FIG. 9, a far field pattern (FFP) depending on a spreading angle of a laser beam emitted from the VCSEL varies with a driving current and temperature rising of the VCSEL element itself. The driving current is controlled to control the light quantity of the laser beam to become a target light quantity during image forming. Further, when the VCSEL element emits a laser beam, the temperature of the VCSEL element rises. Thus, the FFP varies according to these factors.
An optical scanning apparatus discussed in Japanese Patent Application Laid-Open No. 2006-91157 includes a first aperture, restricting a pass of a laser beam before being incident on a half mirror, and a second aperture, restricting a pass of a laser beam after passing through the half mirror (refer to FIG. 1 in Japanese Patent Application Laid-Open No. 2006-91157). Further, another optical scanning apparatus discussed therein includes a first aperture, restricting a pass of a laser beam reflected by the half mirror, and a second aperture, restricting a pass of a laser beam after passing through the half mirror (refer to FIG. 3 in Japanese Patent Application Laid-Open No. 2006-91157). In these optical scanning apparatuses, sizes of openings of the first aperture and the second aperture are different, so that the light quantity ratio between the light quantity of a laser beam passing through the half mirror and reaching the photosensitive member and the light quantity of a laser beam guided to the optical sensor varies according to the variation of the FFP. Thus, even if the light quantity of the laser beam emitted from the VCSEL is controlled based on the result detected by the optical sensor, the light quantity of the laser beam reaching the photosensitive member cannot be controlled to become a target light quantity.
On the other hand, in an optical scanning apparatus discussed in Japanese Patent Application Laid-Open No. 2006-259098, the light quantity ratio can be kept constant by integrating the half mirror and the aperture, even when the FFP varies. However, in an optical scanning apparatus including a configuration illustrated in FIG. 10, in which a laser beam emitted from a semiconductor laser 1001 passes through a lens barrel portion 1003a (a lens tube) of a holding member 1003 holding a collimator lens 1002, a laser beam reflected on the inner surface of the lens barrel portion 1003a is reflected by the half mirror 1004 and is then incident on the optical sensor 1005. On the other hand, a laser beam reflected on the inner surface of the lens barrel portion 1003a and passing through the half mirror 1004 is not incident on a polygon mirror 1006, so that the laser beam does not reach the photosensitive member. Accordingly, the light quantity ratio varies according to the variation of the FFP of a laser beam.